In high availability digital systems using an array of circuit cards with a common back plane interconnect, it is common practice to provide redundant powering of shared circuitry in order to reduce the frequency of system down time due to power supply failure. Power converter circuits providing this function are required to have fast response times so that in the event of a failure of one of the power sources, the others remaining respond to pick up the additional load without bus voltage varying outside of normal operation tolerances, thereby avoiding interruption of service. The sharing of load current has the advantages of reducing circuit reaction time to sudden load increases such as those occurring during a converter failure. Current sharing also reduces stress on individual power converters, thereby increasing power supply MTBF mean time before failure (MTBF) and system availability.
In circuits which do not have current sharing, there can be response time problems when a power supply or source fails or goes offline, since a backup power source or supply must pick up the load quickly. When there is not a current shared redundancy, the secondary source is typically in a saturated off condition. It takes time for the secondary power supply to respond to the increase in load demand. The response time required from a saturated off state to a full power operating state is greater than the response time from half power operating to that of full power operation.
Current sharing is typically accomplished in circuits by using an interconnection between power supplies, typically a current control signal which forces all of the power supplies to operate at approximately the same current level. Such a scheme provides excellent current sharing, but a fault on the current control line can cause the entire system to shut down. Techniques for preventing such a shutdown are very complicated, often including custom integrated circuits (ICs). Current sharing may also be accomplished by adding resistance in series with source outputs. Larger output resistances result in better current balance between sources but result in poorer voltage regulation and power loss, so there are trade offs between regulation and current balance between outputs.
There is a need in the art for current sharing which provides redundant power, balanced current sharing and low loss.